If that is the only 'blemish' you have spotted in your pretty croton, I wouldn't spend one more second worrying about it.

Remember to use your fingers to wiggle down into the potting soil to check the dampness level a few inches deep. The top will always dry out faster. Another excellent method is to take a small wooden dowel rod, sharpened on one end, to insert all of the way to the bottom. Make sure that the rod is dry when you put it in so that you can evaluate the dampness when you pull it out.

I've seen things like it before on my Crotons but unless it starts growing and showing up on more leaves, it's probably just a scar from damage of some kind. Maybe something bit it when it was growing, maybe a substance dripped onto it and killed the cells there -- either way, not a big deal unless it spreads rapidly or onto other leaves.

Crotons are, through generations of breeding, fairly genetically unstable -- so there's even a chance this is just some cells that mutated badly and died at an early stage of the leaf growth and ended up looking bigger after it grew out. You never know with Crotons!